This invention is of an electrochemical machining process for the manufacture of integrally stiffened shell and plate structures such as waffle, ring stiffened and stringer stiffened structures.
Said structures typically comprise a skin integral with numerous stiffening ribs. In the case of shell isogrid and waffle structures for which a flat pattern can be developed, said ribs form respectively triangular and rectangular pockets. In the case of such ring and stringer stiffened structures said ribs are oriented in respectively the circumferential and longitudinal directions. Shells with compound curved surfaces and flat plates may typically be comprised of skins stiffened by a variety of crossing and non-crossing rib stiffening patterns.
Said structures are typically made of aluminum or titanium and are produced by such processes as machining, chemical machining, or investment casting.
In the case of machined structures, the minimum producible skin thickness, typically 0.018 inches, is influenced by the deformation of the skins due to residual or machining induced stresses. Such structures must also include rib blending radii which are equal to or greater than the radius of the end mill used for material removal. Machining tolerance on skin thickness and rib width are typically .+-.0.005 inch.
When chemical machining is used to produce said structures the skin thickness and rib width tolerance are respectively typically .+-.0.008 inch and .+-.0.030 inch. The minimum skin and rib gages is typically 0.020 inch. A single chemical machining cut produces rib-to-rib and rib-to-skin blending radii approximately equal to the depth of cut. A double chemical machining cut at most cuts these blending radii in half and produces a rib cross section having properties approximating that of a machined blade.
Investment castings typically have minimum gages of 0.090 inch, gage tolerances of .+-.0.025 inch and minimum blending radii of 0.125 inch.
Electrochemical machining, applied according to the processes described in this patent, can produce minimum gages, tolerances and blending radii significantly smaller than the best of these features achieved by the above described machining, chemical machining and casting processes.
Electrochemical machining is typically performed by:
applying a variable controlled voltage in the range of zero to 100 volts to the workpiece to be electrochemically machined, PA1 injecting electrolyte, such as salt water solution or sodium nitrate, at pressures in the range of 100 to 3000 psi, into the gap between the shaped and grounded electrode tools and the workpiece and providing back pressure for sustaining this hydrodynamic pressure by restricting but not preventing the flow of electrolyte, PA1 causing the action of the electrolyte hydrodynamic forces to separate the workpiece and cathodic electrode tool, PA1 forcing the electrode against the workpiece and insuring that the electrolyte hydrodynamics action prevents metal to metal contact between the electrode tool and the workpiece, PA1 controlling the electrolyte flow to wash away metal ions formed in the electrolyte before the ions are able to plate the electrode tool, PA1 controlling the electrolyte flow to carry away excess heat generated during the electrochemical machining process, and PA1 controlling the voltage applied to the workpiece as a function of the displacement and force applied to the workpiece.
Metal removal is accomplished by a reverse electroplating or anode depletion or deplating process producing a metallic hydroxide emulsion suspended in the electrolyte. A filter press is used to remove the metal hydroxides produced by the electrochemical machining process and to return the reusable electrolyte to the electrolyte pumping system.
The induction of unidirectional electrolyte flow across the face of the electrode tool requires use of complex sealing schemes. Alternatively, the introduction of electrolyte into the electrode tool results in a hump, bump or protrusion under the electrolyte feeder orifice.
The above described electrochemical machining systems are too complex, costly and technically limited to be applicable to the production of said integrally stiffened structures. This invention overcomes these and other limitations and converts typically undesirable electrochemical machining attributes into vitally useful features.